Email gateway configuration when main email server is outside of firewall

Linux 2.4.8-26mdk
perl5 revision 5.0 version 6 subversion 1
apache 1.3.23

Does anyone have there main email server outside of the firewall. I am
having trouble configuring a way to create tickets via email. My RT is on
a machine within the firewall, and I have been trying to figure out how to
have email forwarded from the outside email server, through the firewall,
and into RT. Does anyone have this configuration, or any suggestions?